Survival of central and peripheral neurons largely depends upon contact with neurotrophins that are released from their target cells (Levi-Montalcini, 1987, EMBO J., 6, 1145-1154; Barde, 1994, Prog. Clin. Biol. Res., 390, 45-56). The neurotrophic effect of neurotrophins is initiated through binding to TrkA, TrkB, or TrkC, the high affinity neurotrophin receptors with tyrosine kinase activity (Patapoutian and Reichardt, 2001, Curr. Opin. Neurobiol., 11, 272-280; Kaplan and Miller, 2000, Curr. Opin. Neurobiol., 10, 381-391). The Trk tyrosine kinases activate the small GTP-binding protein Ras, PI-3K, and PLC, which play an important role in survival of a variety of neurons including cerebellar granule, cortical, hippocampal, sympathetic, and sensory neurons (Borasio et al., 1993, J. Cell Biol., 121, 665-672; Stephens et al., 1994, Neuron, 12, 691-705; Yao and Cooper, 1995, Science., 267, 2003-2006; Nobes et al., 1996, Neuroscience., 70, 1067-1079; Nonomura et al., 1996, Brain Res Dev Brain Res., 97, 42-50; Alcantara et al., 1997, J Neurosci., 17(10), 3623-3633; Hetman et al., 1999, J Biol Chem., 274, 22569-22580; Atwal et al., 2000, Neuron., 27, 265-227).
Neurotrophins enhance neuronal survival by interfering with programmed cell death or apoptosis in the process of normal development (Barde, 1994, Prog. Clin. Biol. Res., 390, 45-56; Deshmukh and Johnson, 1997, Mol. Pharmacol., 51, 897-906). The neuroprotective effects of neurotrophins have been observed in the central neurons subjected to pathological insults. For example, neurotrophins ameliorate degeneration of basal forebrain cholinergic neurons, retinal ganglion neurons, and spinal sensory and motor neurons following axotomy in vivo (Hefti, 1986, J. Neurosci., 6, 2155-2162; Yan et al., 1993, J. Neurobiol., 24, 1555-1577; Mey and Thanos, 1993, Brain Res., 602, 304-317; Morse et al., 1993, J. Neurosci., 13, 4146-4156; Cohen et al., 1994, J. Neurobiol., 25, 953-959; Friedman et al., 1995, J. Neurosci., 15, 1044-1056). Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophins (NT)-4/5 can reduce neuronal death following hypoxic-ischemic injury (Hefti, 1986, J. Neurosci., 6, 2155-2162; Yan et al., 1993, J. Neurobiol., 24, 1555-1577; Mey and Thanos, 1993, Brain Res., 602, 304-317; Morse et al., 1993, J. Neurosci., 13, 4146-4156; Cohen et al., 1994, J. Neurobiol., 25, 953-959; Friedman et al., 1995, J. Neurosci., 15, 1044-1056). BDNF protects dopaminergic neurons from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 6-hydroxy dopamine (Spina et al., 1992, J. Neurochem., 59, 99-106; Frim et al., 1994, Proc. Natl. Acad. Sci. U.S.A., 91, 5104-5108).
The findings above suggest therapeutic potential of neurotrophins for hypoxic-ischemia and various neurodegenerative diseases. However, the beneficial effects of neurotrophins should be compromised with a notion that neurotrophins can exacerbate certain forms of neuronal injury. BDNF, NT-3, or NT-4/5 renders neurons highly vulnerable to deprivation of oxygen and glucose, possibly by enhancing Ca2+ influx and NO (nitric oxide) production through N-methyl-D-aspartate (NMDA) glutamate receptors (Fernandez-Sanchez and Novelli, 1993, FEBS Lett., 335, 124-131; Koh et al., 1995, Science, 268, 573-575; Samdani et al., 1997, J. Neurosci., 17, 4633-4641). BDNF, NGF, and NT-4/5 potentiate neuronal cell necrosis induced by oxidative stress or zinc in cortical cell cultures (Gwag et al., 1995, Neuroreport, 7, 93-96; Park et al., 1998, Neuroreport, 9, 687-690; Won et al., 2000, Neurobiol Dis, 7, 251-259).
Recently, the present inventors have found that neurotrophins can directly induce neuronal cell necrosis in cortical cell cultures and adult rats as well as the potentiation effects of certain neuronal injury (Kim et al., 2002, J. Cell Biol, 159, 821-831). Accordingly, the unexpected neurotoxicity of neurotrophins likely explains failure of clinical trials in neuropathic pain and amyotrophic lateral sclerosis (Apfel et al., 2001, Clin. Chem. Lab. Med, 39(4), 351-61).
Thus, the inventors have delineated mechanisms underlying toxic effects of neurotrophins, investigated drugs for prevention of neurotrophin toxicity, and completed the present invention by developing a method for optimizing therapeutic effects of neurotrophins with anti-oxidants.